A tool head of the above-mentioned type for use in a reamer is known, for example, from international laid-open publication WO 2008/155104 A1. The adjusting device provided there comprises a conical adjusting sleeve which surrounds an adjusting screw in the manner of a floating mounting and the outer face of which interacts with an annular portion of the tool head, so that, overall, a wedge-type mechanism for widening the annular portion is implemented. Force transmission, required for setting purposes, from the adjusting screw to the adjusting sleeve takes place via a bearing face of a screw head provided on the adjusting screw. So that, when the adjusting device is being demounted, the adjusting sleeve can be removed in a simple way from the annular portion surrounding it, the adjusting screw is provided with a pull-out aid in the form of a threaded sleeve, of an annular collar or of a cross pin. The adjusting device therefore has a relatively large number of individual components and is comparatively complicated to manufacture.
Furthermore, it is desirable to have, as compared with the possibilities afforded by the wedge-type mechanism, improved adjustability of the expandable tool head, particularly within the framework of what is known as a fine adjustment with precision in the micrometer range.